1. Field of the Invention
The present invention relates to a positive electrode active material that can be used in an alkaline storage battery and a nickel metal hydride storage battery.
2. Description of Related Art
With the recent spread of portable devices, alkaline storage batteries are demanded to have higher capacity. Particularly, nickel metal hydride storage batteries are secondary batteries, which comprise positive electrodes mainly composed of nickel hydroxide and negative electrodes mainly composed of a hydrogen-absorbing alloy, have spread as secondary batteries of high capacity and high reliability.
Conventional positive electrodes for alkaline storage batteries will be explained below.
The positive electrodes for alkaline storage batteries are roughly classified into two types that are sintered and unsintered. The former type of positive electrode is prepared by sintering a core material such as a punching metal and a nickel powder to obtain a nickel sintered substrate having a porosity of about 80%, impregnating the resulting substrate with an aqueous solution of a nickel salt such as aqueous nickel nitrate solution and then dipping the-substrate in an aqueous alkaline solution, thereby to produce nickel hydroxide in the porous nickel sintered substrate. The positive electrode thus produced has a limited substrate porosity, which makes it difficult to increase the porosity. It is thus impossible to increase the content of the active material to fill in. This difficulty was a limit of conventional storage batteries in improving their capacity.
The latter positive electrodes are those disclosed in, e.g., JP-A-50-36935, which are obtained by filling nickel hydroxide as an active material in a sponge-like three-dimensionally continuous nickel metal-made porous substrate having a porosity of 95% or more. This type has now been widely used for secondary batteries as positive electrodes of high capacity.
For the unsintered positive electrodes of the latter type, it was proposed to fill spherical nickel hydroxide particles in a porous substrate, in terms of the demanded higher capacity. More specifically, the unsintered positive electrodes are obtained by filling spherical nickel hydroxide particles having a particle diameter of several to several tens μm in the porous part (a pore size of approximately 200 to 500 μm) of the sponge-like porous substrate.
Nickel hydroxide particles, which are located around the skeleton of the nickel metal, maintain a conductive network so that charging/discharging response proceeds smoothly. However, the response of nickel hydroxide particles, which are apart from the skeleton, is not satisfactorily smooth.
In order to improve a utilization ratio of the filled nickel hydroxide in the unsintered positive electrodes, a conductive agent is employed in addition to nickel hydroxide as an active material, whereby the spherical nickel hydroxide particles are electrically connected with each other.
Cobalt compounds such as cobalt hydroxide and cobalt monoxide, metallic cobalt, metallic nickel and the like are used as the conductive agent. Thus, it becomes possible to fill the active material at a high density in unsintered type positive electrodes, and the capacity can be increased as compared with the sintered type positive electrodes.
Furthermore, JP-A 8-148145 discloses a method for producing an active material of a positive electrode for high capacity nickel metal hydride storage batteries that are excellent in overdischarge characteristics and meet the market demand for improvement of cycle characteristics, which comprises coating a cobalt compound on an active material nickel hydroxide and subjecting the cobalt compound to an alkali oxidation treatment to convert the compound to a higher order cobalt oxide. JP-A 9-73900 discloses an improvement of the above method.
According to these methods, the nickel hydroxide powders coated with the cobalt compound are sprayed with an aqueous alkaline solution under fluidization or dispersion in the heated air. As a result, it has become possible to make alkaline storage batteries of high energy density, which are improved in utilization ratio of an active material and battery characteristics such as high rate discharge characteristics as compared with the conventional methods in which the cobalt compound is added as an external additive.
Moreover, in alkaline storage batteries, a phenomenon of reduction of charging efficiency occurs when a temperature of the batteries is high. For solving this problem, the electrolyte used in nickel metal hydride storage batteries is optimized. Further, calcium compounds or rare earth oxides such as yttrium oxide and ytterbium oxide that improve the high-temperature charging efficiency are added to positive electrode active materials. This is disclosed, for example, in JP-A-9-92279.